The Complicated Relationship Between Testosterone and Muscle Growth

It's not the levels of testosterone that men have that matters, but rather how sensitive they are to the hormone.

Supplement companies like to make testosterone seem like a miracle hormone for men. Raise it, and you might just turn into a stronger, more energetic, and all-around more amazing version of yourself. If it all sounds too good to be true, that’s mostly because it is. While the promised land of permanently elevated testosterone may appear wonderful in the ads, the actual link between testosterone and muscle growth is not as straightforward as many claim.

Let’s start at the top: Testosterone levels vary naturally from person to person. In the past, I would have told you that more testosterone equals faster muscle growth. That is, if your testosterone levels are hovering around the higher end of the normal range, you’ll have an easier time gaining muscle than someone with naturally lower testosterone levels.

Put differently, the more testosterone you have, the more muscle you gain, even if you lift the same weights and eat the same food. It’s an idea that sounds simple enough, and it seems to make sense. The latest research on testosterone and muscle growth, however, shows that the connection between the two is a lot murkier than was once believed.

We’ve known for some time that any temporary surge in post-training testosterone levels has little impact on the amount of muscle you gain over time. In one study, researchers analyzed data collected from 56 men who took part in a 12-week resistance training program. If the post-exercise change in testosterone levels was important as far as building muscle is concerned, you’d expect to see two things: Guys with the largest testosterone response to training would build the most muscle. And those with the smallest response would build the least muscle.

But when they looked at the data, the researchers could find no significant link between the exercise-induced rise in testosterone levels and gains in muscle mass. Drilling further down into the results, subjects in the study were divided into responders (men who built the most muscle) and non-responders (those who built the least muscle). The hormonal responses of those who made the fastest gains in size and strength were no different than those who made the slowest gains.

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More interesting still, the amount of testosterone you have at rest—in healthy young men, at least—doesn’t appear to have much to do with muscle growth either. In fact, the latest science shows that guys who built the most muscle after 12 weeks of weight training weren’t the ones with the highest testosterone levels, but the ones with more androgen receptors.

Why do androgen receptors matter? For testosterone to do all the things we know and love as far as muscle growth is concerned, it needs to interact with muscle tissue. And it does that via androgen receptors. One of the ways androgen receptors respond to a hormone like testosterone is by signaling muscle cells to increase the rate at which new muscle protein is laid down. Over time, this increase in muscle protein synthesis leads to bigger, stronger muscles.

The guys who built the most muscle didn’t have higher testosterone levels, but they did have more androgen receptors in their muscles than subjects who gained the least muscle. This meant their bodies were able to make better use of the testosterone that was available.

In other words, it isn’t testosterone levels that predict muscle gains, but how sensitive your muscles are to that testosterone. Think of it like a car, with testosterone as the engine and androgen receptors as the tires. If you’ve got a powerful V8 engine but cheap tires, you’re not going to be able to make full use of that power. Likewise, you need those androgen receptors to take full advantage of the testosterone that’s in your system.

All of which begs the question: If testosterone levels aren’t linked to muscle growth, why does injecting yourself with testosterone make your muscles grow faster? Anabolic drugs work because they take your testosterone levels outside of their normal physiological range and keep them there, night and day, for weeks, months, and sometimes even years on end.

A team of Californian scientists found that guys who combined three days a week of weight training with weekly testosterone injections gained a whopping 13 pounds of muscle in just 10 weeks. The amount of testosterone they were given—600 milligrams per week—is a lot, around six times higher than the dose usually given to men on hormone replacement therapy. It was enough to raise their testosterone levels by more than 600 percent.

In other words, the injections took testosterone levels well outside of their normal physiological range, which is why they had such a big impact on muscle growth. Even if you could raise your testosterone levels by 10, 20, or 30 percent, it’s a drop in the bucket compared to what you’re getting from drugs.

It’s a similar story when it comes to the link between testosterone and aggression. The standard view is that the more testosterone you have, the more aggressive you are, while less testosterone equals less aggression. But it’s not that straightforward, and a simple one-to-one link between testosterone and aggression doesn’t hold.

“Remove someone’s testes and the frequency of aggressive behavior is likely to plummet,” says Robert Sapolsky, a Stanford University neurobiologist. “[If you] reinstate precastration levels of testosterone by injecting the hormone, precastration levels of aggression typically return.” But there’s a twist: If you only restore 20 percent of the person’s normal testosterone levels, normal levels of aggression still return. Double their normal level of testosterone, however, and levels of aggression don’t increase above the precastration level.

What this tells us, Sapolsky says, is that you need some testosterone around for normal aggressive behavior. When there’s no testosterone, aggression plummets. Quadruple the normal level of testosterone, and aggression goes up. But anywhere from roughly 20 percent of normal to twice the normal level and it’s all the same; the brain can’t distinguish among this wide range of basically normal values.”

Why does all this matter? For one thing, testosterone is what endocrinologists call a “permissive” hormone. That is, once there’s enough testosterone in your system to “permit” your muscles to grow, it’ll take a gargantuan spike to increase that growth rate significantly.

If your testosterone levels are well below normal, you’ll find that gaining muscle is a slow process. When levels are several hundred percent higher than normal, your muscles are going to grow a lot more quickly. Fluctuations in testosterone within the normal physiological range, however, aren’t going to do much of anything, and trying to raise your testosterone levels naturally is unlikely to have much of an impact on the speed at which you gain muscle.